Exposure Difference When Using FE Lock

[jazzone]

I've noticed that when using my 580EX ii in ETTL mode with my 40D, that if I first press the <*> button (FE Lock) I get a higher (and generally more accurate) exposure (+2/3 to +1 EV) than when I don't. I thought the flash always fires a pre-flash immediately before the main flash to establish proper flash exposure so I was surprised there's a difference. Has anyone experienced this and does anyone know why this happens?

[Pip]

That's a new one on me! Maybe Syl will be able to help next time he's on here, there's not much about our units he's not come across.

[glsevans]

I have noticed this on my 40D as well. Do you have an off-center focus point selected when this happens to you? As it was explained to me that the FE lock <*> button spot meters on the center focus point so you may recompose and shoot, whereas a normal exposure uses evaluative metering (if selected) weighting it upon the selected focus point.

[Syl Arena]

FE Lock fires a pre-flash and then meters it in the center circle of the viewfinder. FEL is great to use when you've got an exposure dialed in (perhaps with EC and FEC in action) and you want to make alternate compositions -- shoot wide vs. zoomed in or vertical vs. horizontal. If you return to your original composition and hit FEL each time before you recompose for the alternate shots, then your E-TTL exposures will be very similar.

It's very difficult to know why any camera appears to be more accurate with FEL than without because the evaluative part of E-TTL metering is always based on what the camera is seeing. If you shoot back-lit scenes a lot, then FEL will be a big help. For more general shooting, it's virtually impossible to say.